Tangetially related: For the people who don’t know, I found out recently that x!! isn’t the same as (x!)! (repeated factorial), in fact, !! is a LOT less big than repeated factorial.
For example, while 30!! is 4.286 x 10^16 (so a number with 17 digits), doing 30! and then ! the result of that, would be a number with an unfathomable 10^33 digits.
n!! is its own operator called the “double factorial” and is even smaller than the regular !, because it’s the product of only the odd numbers up to n.
Edit: escape characters
No clue. Wolfram Alpha interprets it as (10!!)!, so I assume !!! isn’t an established operator, unlike the very unintuitive !!.